We present a theoretical study of the optical generation of one-photon spin and current injection onto In-adsorbed Si(111) surfaces with $4\times 2$ and $8\times 2$ reconstructions. The spin injection, under incidence of circularly polarized light into nonmagnetic semiconductors, creates spin-polarized electrons in the conduction bands. The current injection is a nonlinear second-order effect that is allowed in materials without inversion symmetry. In bulk centrosymmetric crystals, the optical injection of current can only be observed at the surface wherein the inversion of symmetry might be broken. We report calculations for the degree of spin polarization and current-injection spectra which are calculated in a full electronic band structure scheme at the level of GW scissor-energy correction. Our results show an anisotropic behavior of the spin- and current-injection optical response. We obtain maximum percentages of the degree of spin polarization of 30% and 35% for the $4\times 2$ and $8\times 2$ surface reconstructions, respectively. It is also possible to optically generate injection current coming mainly from the first two top layers on both In-adsorbed surface reconstructions.

We observe quasi-long-range coherence in a two-dimensional condensate of exciton-polaritons. Our measurements confirm that the spatial correlation algebraically decays with a slow power law, whose exponent quantitatively behaves as predicted by the Berezinskii-Kosterlitz-Thouless theory. The exciton-polaritons are created by nonresonant optical pumping of a microcavity sample with embedded GaAs quantum wells at liquid helium temperature. Michelson interference is used to measure the coherence of the photons emitted by decaying exciton-polaritons.

We investigate the effect of potential barriers in the form of step edges on the scattering properties of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$(111) topological surface states by means of large-scale ab initio transport simulations. Our results demonstrate the suppression of perfect backscattering, while all other scattering processes, which do not entail a complete spin and momentum reversal, are allowed. Furthermore, we find that the spin of the surface state develops an out-of-plane component as it traverses the barrier. Our calculations reveal the existence of quasibound states in the vicinity of the surface barriers, which appear in the form of an enhanced density of states in the energy window corresponding to the topological state. For double barriers we demonstrate the formation of quantum well states. To complement our first-principles results we construct a two-dimensional low-energy effective model and illustrate its shortcomings. Our findings are discussed in the context of a number of recent experimental works.

When Dirac electrons on the surface of a topological insulator are gapped, the resulting quantum anomalous Hall effect leads to universal magneto-optical Faraday and Kerr effects in the low-frequency limit. However, at higher frequencies different excitations can leave their own fingerprints on the magneto-optics and can therefore be probed. In particular, we investigate the role of localized in-gap states—which inevitably appear in the presence of charged impurities—on these higher-frequency magneto-optical effects. We have shown that these states resonantly contribute to the Hall conductivity and are magneto-optically active. These in-gap states lead to peculiar resonant signatures in the frequency dependence of the Faraday and Kerr angles, distinct in character to the contribution of in-gap excitonic states, and they can be probed in ellipsometry measurements.

In order to gain insight into the connection between the vibrational dynamics and the atomic-level Green-Kubo stress correlation function in liquids, we consider this connection in a model crystal instead. Of course, vibrational dynamics in liquids and crystals are quite different and it is not expected that the results obtained on a model crystal should be valid for liquids. However, these considerations provide a benchmark to which the results of the previous molecular dynamics simulations can be compared. Thus, assuming that vibrations are plane waves, we derive analytical expressions for the atomic-level stress correlation functions in the classical limit and analyze them. These results provide, in particular, a recipe for analysis of the atomic-level stress correlation functions in Fourier space and extraction of the wave-vector and frequency-dependent information. We also evaluate the energies of the atomic-level stresses. The energies obtained are significantly smaller than the energies previously determined in molecular dynamics simulations of several model liquids. This result suggests that the average energies of the atomic-level stresses in liquids and glasses are largely determined by the structural disorder. We discuss this result in the context of equipartition of the atomic-level stress energies. Analysis of the previously published data suggests that it is possible to speak about configurational and vibrational contributions to the average energies of the atomic-level stresses in a glass state. However, this separation in a liquid state is problematic. We also introduce and briefly consider the atomic-level transverse current correlation function. Finally, we address the broadening of the peaks in the pair distribution function with increase of distance. We find that the peaks' broadening (by $\approx 40\%$) occurs due to the transverse vibrational modes, while contribution from the longitudinal modes does not change with distance.

Electron energy loss spectra enable a detailed quantification of the electronic loss mechanisms in a target solid, particularly in the low-energy region dominated by plasmon excitations. Models of the electronic response in condensed-matter systems are usually derived from free-electron gas or jellium models, which commonly neglect to account for the lifetime broadening of individual plasmon and single-electron excitations in a constrained, physical manner. This can lead to potentially significant errors in electron energy loss spectra and electron inelastic mean-free-path (IMFP) calculations. We develop a toy model of plasmon and single-electron excitations that incorporates lifetime broadening for each excitation in an energy- and momentum-dependent fashion. The model is physically constrained using optical and electronic sum rules. We demonstrate the necessity of asymmetric excitation broadening, and show that causally permitted variations in the broadening function can have a significant impact on the dielectric response of the material. Our developments are applied to molybdenum, and compared with previous modeling and high-precision experimental results for the IMFP at electron energies below 120 eV.

We present a detailed investigation of the magnetic properties of the spin-$\frac{3}{2}$ system ${\mathrm{Li}}_2\mathrm{Co}{\left({\mathrm{WO}}_4\right)}_2$ by means of magnetic susceptibility and specific heat. Our experimental results show that in ${\mathrm{Li}}_2\mathrm{Co}{\left({\mathrm{WO}}_4\right)}_2$ short-range antiferromagnetic (AFM) correlations appear near ${\chi }_{\max }\sim 11\mathrm{K}$ and two successive long-range AFM phase transitions are observed at $T_{N1}\sim 9$ and $T_{N2}\sim 7\mathrm{K}$. The frustration factor $\left|\Theta \right|/T_{N1}\sim 3$ indicates that the system is moderately frustrated, which is identifiable by the broken triangular symmetry within both $ab$ and $bc$ planes for the triclinic crystal structure. The magnetic isotherm at temperatures below $T_{N2}$ shows a field-induced spin-flop transition, and a complete $H\text{−}T$ phase diagram for the two-step AFM system is mapped. Ab initio band-structure calculations suggest that the strongest exchange coupling does not correspond to the shortest Co-Co distance along the $a$ axis but rather along the diagonal direction through a Co-O-W-O-Co supersuperexchange path within the $bc$ plane.

We study the magnetization dynamics in ferromagnet-insulator-ferromagnet and ferromagnet-insulator-normal metal ultrasmall tunnel junctions, and the associated voltage drop in the presence of an electromagnetic environment assisting the tunneling processes. We show that the environment strongly affects the resulting voltage, which becomes a highly nonlinear function of the precession cone angle $\theta$. We find that voltages comparable to the driving frequency $\omega$ can be reached even for small precession cone angles $\theta$, in stark contrast to the case where the environment is absent. Such an effect could be useful for the detection local magnetization precessions in textured ferromagnets or, conversely, for probing the environment via the magnetization dynamics.

Here we calculate the pairing interaction and the $k$ dependence of the gap function associated with the nematic charge fluctuations of a ${\mathrm{CuO}}_2$ model. We find that the nematic pairing interaction is attractive for small momentum transfer and that it gives rise to $d$-wave pairing. As the doping $p$ approaches a quantum critical point, the strength of this pairing increases and higher $d$-wave harmonics contribute to the $k$ dependence of the superconducting gap function, reflecting the longer range nature of the nematic fluctuations.

We use element-resolved IR-pump/extreme ultraviolet-probe experiments to disentangle the ultrafast interplay of the magnetic sublattices of an ordered crystalline magnetic alloy. As a paradigmatic example, we investigate the case of the FeRh alloy, which shows a delayed response for the different components. Furthermore, a detailed time-resolved magneto-optic study shows that the data can be analyzed by only assuming Elliot-Yafet-like scattering, as the underlying mechanism for ultrafast demagnetization, resulting in an unexpected nonmonotonic dependence of the spin-flip rate, as a function of quenching.

The structural and polar properties of $\mathrm{BiFe}{\mathrm{O}}_3$ at finite temperature are investigated using an atomistic shell model fitted to first-principles calculations. Molecular dynamics simulations show a direct transition from the low-temperature $R3c$ ferroelectric phase to the Pbnm orthorhombic phase without evidence of any intermediate bridging phase between them. The high-temperature phase is characterized by the presence of two sublattices with opposite polarizations, and it displays the characteristic double-hysteresis loop under the action of an external electric field. The microscopic analysis reveals that the change in the polar direction and the large lattice strains observed during the antiferroelectric-ferroelectric phase transition originate from the interplay between polarization, oxygen octahedron rotations, and strain. As a result, the induced ferroelectric phase recovers the symmetry of the low-temperature $R3c$ phase.

We study Ising pyrochlores by means of Monte Carlo simulations. We cover a set of exchange constants ranging from the frustrated ferromagnetic case (spin-ice) to the fully-ordered “all-in–all-out” antiferromagnet in the dipolar model, reinterpreting the results—as in an ionic system—in terms of a temperature vs magnetic charge density phase diagram. In spite of its spin nature and the presence of both double and single nonconserved magnetic charges, the dipolar model gives place to a phase diagram which is quite comparable with those previously obtained for on-lattice systems of electric charges, and on spin ice models with a conserved number of single magnetic charges. The contrast between these systems, to which we add results from the nearest-neighbors model, put forward other features of our phase diagram—notably, a monopole fluid with charge order at high monopole densities that persists up to arbitrarily high temperatures—that can only be explained taking into account construction constraints forced by the underlying spin degrees of freedom.

We report the appearance of ferroelectricity below $T_S=18$ K, at which short-ranged spiral spin orders, suggesting that ${\mathrm{MnCr}}_2{\mathrm{O}}_4$ is an improper multiferroic material. Linear heat capacity data and zero-field-cooled memory effect verify reentrant spin-glass-like state below $T_S$. Synchrotron x-ray diffraction studies confirm significant magnetoelastic coupling at long-range ferrimagnetic order ($T_N=42$ K) and at $T_S$. We argue that the release of topological spin frustration, produced due to pyrochlore structure of Cr sublattice, is manifested through the significant magnetoelastic effect at $T_N$ and $T_S$ and holds the key for spiral spin order driven ferroelectricity in ${\mathrm{MnCr}}_2{\mathrm{O}}_4$.

The double exchange (DE) model is systematically applied for studying the coupling between ferroelectric (FE) and magnetic orders in several prototypical types of multiferroic manganites. The model itself was constructed for the magnetically active Mn $3d$ bands in the basis of Wannier functions and includes the effect of screened onsite Coulomb interactions in the Hartree-Fock approximation. All model parameters were derived from the first-principles electronic-structure calculations. The essence of our approach for the FE polarization is to use the Berry-phase theory, formulated in terms of occupied Wannier functions, and to evaluate the asymmetric spin-dependent change of these functions in the framework of the DE model. This enables us to quantify the effect of the magnetic symmetry breaking and derive several useful expressions for the electronic polarization $\mathbf{P}$, depending on the relative directions of spins. The spin dependence of $\mathbf{P}$ in the DE model is given by the isotropic correlation functions ${\mathbf{e}}_i·{\mathbf{e}}_j$ between directions of neighboring spins. Despite formal similarity with the magnetostriction mechanism, the magnetoelectric coupling in the proposed DE theory is not related to the magnetically driven FE atomic displacements and can exist even in compounds with the centrosymmetric crystal structure, if the spatial distribution of ${\mathbf{e}}_i·{\mathbf{e}}_j$ does not respect the inversion symmetry. The proposed theory is applied to the solution of three major problems: (i) the magnetic-state dependence of $\mathbf{P}$ in hexagonal manganites, using ${\mathrm{YMnO}}_3$ as an example; (ii) the microscopic relationship between canted ferromagnetism and $\mathbf{P}$ in monoclinic ${\mathrm{BiMnO}}_3$; (iii) the origin of FE activity in orthorhombic manganites. Particularly, we will show that for an arbitrary noncollinear magnetic structure, propagating along the orthorhombic $\mathbf{b}$ axis and antiferromagnetically coupled along the $\mathbf{c}$ axis, the polarization is induced by an inhomogeneous distribution of spins and can be obtained by scaling the one of the E-type antiferromagnetic (AFM) phase with the prefactor depending only on the relative directions of spins and being the measure of this spin inhomogeneity. This picture works equally well for the twofold (${\mathrm{HoMnO}}_3$) and fourfold (${\mathrm{TbMnO}}_3$) periodic manganites. The basic difference is that, even despite some spin canting of the relativistic origin and deviation from the collinear E-type AFM alignment, the twofold periodic magnetic structure remains strongly inhomogeneous, which leads to large $\mathbf{P}$. On the contrary, the fourfold periodic magnetic structure can be viewed as a moderately distorted homogeneous spin spiral, which corresponds to much weaker $\mathbf{P}$.

${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ has drawn much attention as the leading candidate to be the first topological superconductor and the realization of coveted Majorana particles in a condensed matter system. However, there has been increasing controversy about the nature of its superconducting phase. This study sheds light on ambiguity in the normal-state electronic state by providing a complete look at the quantum oscillations in magnetization in ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ at high magnetic fields up to 31 T. Our study focuses on the angular dependence of the quantum oscillation pattern in a low carrier concentration. As the magnetic field tilts from the crystalline $c$ axis to the $ab$ plane, the change of the oscillation period follows the prediction of the ellipsoidal Fermi surface. As the doping level changes, the 3D Fermi surface becomes quasicylindrical at high carrier density. Such a transition is potentially a Lifshitz transition of the electronic state in ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$.

We present a density-functional-theory analysis of crystalline and amorphous Zn- and Sn-based oxide systems which focuses on the electronic defect states within the band gap. A comparison of these electronic levels reveals that the hybrid functionals PBE0, HSE06, or B3LYP agree with a self-interaction corrected (SIC) local-density-approximation functional on occupied defect levels when similar treatments of the self-interaction are considered. However, for unoccupied levels, the hybrid functionals and the SIC approach lead to very different predictions. We show that a prerequisite for the determination of the energetic position of subgap states in these oxides is that a functional needs to predict correctly the electronic band structure over a wide energy range and not just close to the band gap. We conclude that for accurate defect levels, an adequate treatment of the self-interaction problem is required especially in the presence of nearby metal-metal interactions.

We study the effect of dimerization (due to, e.g., spin-Peierls instability) on the phase diagram of a frustrated antiferromagnetic spin-1/2 ladder, with weak transverse and diagonal rung coupling. Our analysis focuses on a one-dimensional version of the model (i.e., a single two-leg ladder) where we consider two forms of dimerization on the legs: columnar dimers (CDs) and staggered dimers (SDs). We examine in particular the regime of parameters (corresponding to an intermediate $XXZ$ anisotropy) in which the leg dimerization and the rung coupling terms are equally relevant. In both the CD and SD cases, we find that the effective field theory describing the system is a self-dual sine-Gordon model, which favors ordering and the opening of a gap to excitations. The order parameter, which reflects the interplay between the leg and rung dimerization interactions, represents a crystal of 4-spin plaquettes on which longitudinal and transverse dimers are in a coherent superposition. Depending on the leg dimerization mode, these plaquettes are closed or open, however both types spontaneously break reflection symmetry across the ladder. The closed plaquettes are stable, while the open plaquette order is relatively fragile and the corresponding gap may be tuned to zero under extreme conditions. We further find that a first-order transition occurs from the plaquette order to a valence bond crystal (VBC) of dimers on the legs. This suggests that in a higher-dimensional version of this system, this variety of distinct VBC states with comparable energies leads to the formation of domains. Effectively one-dimensional gapless spinon modes on domain boundaries may account for the experimental observation of spin-liquid behavior in a physical realization of the model.

The observation of the quantum spin Hall effect without the need of an external magnetic field in HgTe/CdTe heterostructures triggered the study of materials exhibiting persistent spin-polarized electronic currents at their interfaces. These Dirac-like spin states are predicted to be topologically protected against perturbations preserving time-reversal symmetry. However, nonmagnetic (time-reversal preserving) perturbations will certainly affect these interface states. In this work, the density functional theory is used to characterize the topologically protected states of the (001) HgTe/CdTe heterostructure as well as to understand the influence of external adiabatic parameters as pressure and electric fields on these states. The intrasite Hubbard U term is seen to be important to correctly describe the HgTe bulk band structure. It is shown that, differently from the three-dimensional topological insulators, the HgTe/CdTe interface states present fully in-plane Rashba-like spin texture. Further, biaxial external pressures and electric fields perpendicular to the interfaces are seen to change the energetics and dispersion of the protected states, modifying the energy ordering of the crossing of the polarized interface states inside the band structure, and altering their Fermi velocities while not changing the topological quantum phase. These adiabatic variables can then be used to tune the topologically protected states with respect to the Fermi level.